Traditionally, an efficient power amplifier (PA) operating in a saturated mode in one of class C, class D, class E, or class F is provided with a series modulator that is a variable modulator to the supply voltage. This variable supply input voltage from the power modulator may proportionally affect the output voltage and thus, the Radio Frequency (RF) of the power amplifier. This well-known variable input voltage technique may be known as High Level Modulation (HLM) or also as envelope elimination and restoration (EE&R) where the PA power supply is modulated with a waveform proportional to the RF envelope as described in U.S. Pat. No. 2,903,518 to Kahn.
Overall system inefficiencies, however, may be a primary negative impact of this technique. Notwithstanding the use of a well-known switch mode power amplifier, the creation of an audio waveform for the envelope modulation may be an inherently inefficient process limited to an efficiency of about 90%. These overall system efficiencies may be a combination of the efficiencies in the drain modulator and the PA.
Amplitude modulation has been applied to the output of a Class-D power amplifier using EE&R techniques. Typically, this power modulation function has been performed by a pulse-width modulation (PWM) amplifier as described in U.S. Pat. No. 3,440,566 to Swanson. Unfortunately, this PWM amplifier technique adds additional complexity and power inefficiency to the transmitter, and complex design inherencies may lead to eventual waveform distortion. In Class D operation, two devices may complementarily operate in a totem pole or push-pull methodology.
In Class F operation, two devices may traditionally operate in conjunction employing harmonic tuning to attempt to create a square wave voltage at the drain of the switching amplifier.
In Class E operation, one efficiency goal may be to obtain a drain voltage waveform representing a damped sinusoid. Well-known implementation typically may follow the teaching of U.S. Pat. No. 3,919,656 to Sokal, et. al. Sokal teaches a switched RF device embedded within a band pass filter of particular construction. This construction may traditionally comprise a shunt capacitor associated within the switch either added to or comprised wholly of the drain source capacitance parasitics of the device. Alternatively, construction may comprise a required external capacitance to form the desired reactance at the frequency of choice. In addition, a series tuned band pass circuit may allow passage of the fundamental frequency of operation while rejecting or presenting high impedance to all harmonics thereof, and a load that is represented by a resistance at the particular frequency of operation.
A digital modulation scheme has been described in U.S. Pat. No. 4,580,111 to Swanson, wherein a transmitter comprised of a sufficient quantity of discrete power amplifier modules may modulate the output amplitude of the RF waveform by selecting a number of these power amplifier modules to be disabled at any given time. This technique may eliminate some of the complexity and efficiency penalty inherent with an EE&R solution, but unfortunately requires a sufficient quantity of modules to reduce the envelope quantization below an acceptable level.
A further modulation technique is known as outphasing as described in U.S. Pat. No. 2,269,518 to Chireix, et. al., wherein the transmitter is subdivided into two halves, which are then complementarily phase modulated around the instantaneous RF carrier signal. The vector sum of these two RF waveforms is infinitely variable to zero, and, depending on the configuration of the two power amplifier sections, may be comparable in efficiency to an EE&R solution. A benefit of the outphasing system is that, unlike digital transmitter systems, it may be scalable downward to a mere two PA modules, permitting operation in lower-powered applications. Because the modulation takes place in the phase/frequency domain of the drive signal, there is no intrinsic bandwidth limitation as there is with a PWM-modulated EE&R transmitter architecture. However, outphasing had found limited success to date as applied to the various classes of PAs requiring a sacrifice of output power to achieve maximum efficiency or vice versa.
Chireix teaches a configuration in which two independent PAs are operated with outputs combined in such manner that the net output Voltage (V) is the vector sum of the RF signal generated by each individual PA. Each PA is capable of operation with independent phase control with respect the other. Consequently, when both PAs are operated in phase the output adds constructively, creating maximum output power. Similarly, when the amplifiers are operated out of phase, there exists a situation of maximum destructive interference and consequently zero output power. Thus, at any phase condition between 1) perfectly in phase and 2) perfectly out of phase, the system may produce intermediate levels of output amplitude. Chireix modulation has provided some success using two class C transmitters and has been adapted with limited success with class D solid state amplifiers.
The class E specific requirement of maintaining an output impedance that creates the desired optimal drain voltage waveform has created a barrier to implementation of Chireix modulation in the class E environment. Significant deviations from an optimal phase (e.g. outphasing) may create a mutual load modulation on each respective power amp in such a way that the impedances presented to each amp are separate and generally not benign to the operation of the amplifiers. Non-benign impedance may directly influence waveform control since class E may be more directly sensitive than some of the other classes to design impedance.
Conventional class E power amplifier RF tuning may involve feeding the drain bias (or DC supply voltage) to the amplifier through a large value RF choke. Such conventional choke methods may prove operationally successful however, limitations remain under conventional configurations concerning achieving maximum power output alongside maximum efficiency.
Additionally, a traditional class E limitation may include a sacrifice of power for efficiency. As the frequency of operation for a fixed tuned class E power amplifier is increased, its power output diminishes dramatically related to the resonant frequency of the series LC in the output network.
Bandwidth, power, and phasing limitations of the Power Modulator in turn may negatively affect the linearity and maxim modulation bandwidth of the transmitter itself. In consideration of efficiency limitations combined with bandwidth and power limitations, the current invention brings a novel approach to overcome these challenges.